1) Field of the Invention
This invention relates generally to fabrication of a semiconductor device and more particularly to a method for repairing the shifter layer of an alternating phase shift mask (APSM) by creating an equivalent shifter film both in transmittance and in phase angle transformation.
2) Description of the Prior Art
Photolithography is a well known technique used to apply a pattern (such as a circuit pattern) onto the surface of a workpiece (such as semiconductor chip, wafer, or substrate), and which is also capable of producing very small, intricate patterns for many other applications. Traditional photolithography involves applying electromagnetic radiation through a transparent mask (such as quartz) having an opaque pattern (such as chrome) formed thereon such that the light or radiation passes through the transparent mask, but not through the opaque pattern. The opaque pattern is transferred to the semiconductor structure when the light (electromagnetic radiation) passes through the non opaque areas of the mask (openings) onto a photosensitive material (such as photoresist) which is applied to the surface of the semiconductor structure. The photosensitive material is exposed to a solution which dissolved those portions which have been exposed to light, leaving photosensitive material in the pattern from the mask. (Photosensitive materials are also available wherein the exposed areas remain and the unexposed areas are removed.)
A recent improvement in photolithography technique is the alternating phase shifting mask (APSM), wherein alternating areas of the mask which allow light transmission have a phase shifting layer applied thereon, such that the light is shifted out of phase from the adjacent area. The electromagnetic radiation that passes through the phase shifting openings interferes destructively with the electromagnetic radiation that passes through the non phase shifting openings. This destructive interference reduces pattern distortion caused by reflected and deflected electromagnetic radiation, providing improved resolution between exposed and unexposed areas.
Although APSMs provide significant resolution and/or dimensional control enhancements over traditional (binary) masks, they are often difficult or impossible to repair. This is because any repair to the phase shift layer (shifter layer) of an APSM must meet both the transmittance and phase angle transformation requirements of the damaged or missing shifter layer. Typical shifter layer defects include pinholes, missing shifter layer, or misplaced shifter layer. If the APSM can not be repaired it typically must be replaced in its entirety. Since the probability of fabricating a completely defect free APSM is low, especially for complex state-of-the-art circuits, unacceptable mask costs and delays can result.
Much effort has gone into solving this APSM defect problem. One method for repairing phase shift layer defects requires fabricating a separate repair mask for the defect area, thereby increasing the number of photolithography steps and masks required, resulting in increased processing cost and cycle time. Another method for repairing phase shift layer defects requires fabrication of an APSM with extra phase shift layers which cancel each other out if left in place, and which can be removed to repair missing shifter layers and pinholes. However, mask cost and cycle time is increased even for APSMs without shifter layer defects. Another problem with this method is that it may adversely affect the depth of focus. Also, this method requires greater control over shifter layer thickness to achieve the same phase shift accuracy since three shifter layers are used, and therefore three shifter layer errors, are incurred.
The importance of overcoming the various deficiencies noted above is evidenced by the extensive technological development directed to the subject, as documented by the relevant patent and technical literature. The closest and apparently more relevant technical developments in the patent literature can be gleaned by considering the following patents.
U.S. Pat. No. 5,795,685 (Liebmann et al.) show a method for repairing phase shifting masks by creating a second mask which contains phase shifters, removing the circuitry pattern from the defective areas of the first mask, and copying the circuitry pattern from the defective areas of the first mask onto the second mask.
U.S. Pat. No. 5,964,301 (Nara et al.) teaches a method for repairing a photomask using a chemical etch to remove a residual defect (opaque area in an area that is supposed to be clear). This invention does not address shifter layer defects.
U.S. Pat. No. 5,272,024 (Lin) shows a method of fabricating a repairable phase shifting mask and a method for repairing said mask. In this invention a stack of three phase shifting layers wherein the first and third layer have a phase shift, in radians, of xcex8 (the desired phase shift) and the middle layer has a phase shift, in radians, of 2xcfx80xe2x88x92xcex8. Missing phase shifters, unwanted phase shifters and defective phase shifters can then be repaired by removing one or two layers depending upon whether the phase shifting status has to be retained or altered.
U.S. Pat. No. 5,582,939 (Pierrat) shows a method for fabricating a defect-free phase shifting mask using an etch stop layer under the phase shift layer wherein the etch stop layer is chemically different from the phase shift layer. Defects, such as bump defects in the phase shift area can be removed by etching using the etch stop layer to endpoint the etch process. This invention does not address missing shifter layer or shifter layer pinhole defects.
U.S. Pat. No. 5,384,219 (Dao et al.) discloses a method for making identical inverted phase shifted features.
U.S. Pat. No. 5,881,125 (Dao) shows an attenuated phase shifting reticle using sub-resolution pattern.
U.S. Pat. No. 5,882,823 (Neary) shows a method for repairing an alternating phase shifting mask with a pinhole defect by etching the shifter layer surrounding the defect and by etching the adjacent substrate using FIB sputtering to maintain the 180 degree phase shift between adjacent regions.
U.S. Pat. No. 5,554,465 (Watanabe) and U.S. Pat. No. 5,443,931 (Watanabe) disclose methods for repairing shifter defects using a polymer with a siloxane-bond structure.
It is an object of the present invention to provide a method for repairing shifter layer defects in an alternating phase shifting mask wherein the transmission and phase shift angle are equivalent to a normal phase shifting layer.
It is another object of the present invention to provide a method for repairing a shifter layer defect in an alternating phase shifting mask which does not require additional masks or a mask containing multiple phase shifting layers regardless of the presence or absence of shifter layer defects.
It is another object of the present invention to provide a method for repairing a shifter layer defect in an alternating phase shifting mask which provides a depth of focus comparable to a normal phase shifting layer.
It is yet another object of the present invention that the repair method according to the preceding objects can be applied to an attenuated phase shift mask, as well as an alternating phase shift mask.
To accomplish the above objectives, the present invention provides a method for repairing shifter layer defects such as a missing shifter layer or pin holes in a shifter layer by forming an equivalent shifter layer with the same transmittance, phase shift angle, and depth of focus as an original DUV APSM shifter layer. The key to this invention is a novel two step process to form an equivalent shifter layer with about the same light transmittance and phase angle shift as an original, non-defective shifter layer. For a DUV APSM, transmittance is typically about 6% and phase angle shift is about 180 degrees. The first step is to etch the quartz substrate in a focus ion beam repair machine, using XeF2 gas, to cause a leading phase angle shift. The second step is to deposit an equivalent shifter layer in the focus ion beam repair machine, using a carbon-based gas. When the equivalent shifter layer has about the same transmittance as the original shifter layer (e.g. 6%), the phase angle is lagging less than 180 degrees. The leading phase angle shift caused by etching the quartz substrate and the lagging phase angle caused by the equivalent shifter layer combine to produce a phase angle 180 degrees leading.
The present invention provides considerable improvement over the prior art. The key advantages of the present invention are that shifter layer defects of a phase shifting mask can be repaired so that the equivalent shifter layer provides the same transmittance, phase shifting angle, and DOF as an original non-defective shifter layer. Also, the present invention does not require multiple masks or a mask with multiple phase shifting layers.